We previously provided evidence for the descending inhibitory model of opiate analgesia by studying the lack of effect of morphine on consequences of aversive brain stimulation. In the present renewal application, we propose to test effects of endogenous opiates (enkephalins and endorphins) injected intracranially in periacqueductal central gray on aversive central stimulation effects, and to compare these actions of endogenous opiates with effects of morphine. The objective is to help clarify the physiological role of endogenous opiates and to continue testing models of morphine's inhibitory effect on pain.